1. Field of the Invention
The present invention relates to a sub-harmonic mixer, and more particularly, to a direct-conversion sub-harmonic mixer which is embodied to improve isolation characteristics of an RF signal and a double local oscillator (2LO) signal.
The present research was performed as a part of the IT New Growth Engine Business Planning by the Ministry of Information and Communication (South Korea) and the Institute for Information Technology Advancement (South Korea) [2005-S-039-02, SoP for 60 GHz Pico Cell communication].
2. Discussion of Related Art
Generally, in a wireless communication system using an electric wave, a frequency converter coverts a radio frequency (RF) signal having very low power which is input through an antenna and a local oscillator (LO) signal output from a local oscillator in the system into a baseband frequency signal corresponding to a frequency difference between the two signals. Such frequency conversion may be simply performed by a nonlinear circuit multiplying two signals, and thus the frequency converter generally includes a core functioning as an analog multiplier, input/output buffers, a filter, and so on.
A general theory and conventional technology of a frequency converter circuit are disclosed in “RF Microelectronics” (Prentice Hall, 1998) by B. Razavi, and “The Design of CMOS Radio-Frequency Integrated Circuits” (Cambridge University Press, 1998) by Thomas Lee.
A 0-IF system has been popular in recent times, wherein “0-IF” means a direct conversion method without using an intermediate frequency (IF), that is, the IF is 0 Hz. This method is opposite to a super heterodyne method which is currently widely used.
That is, this is a method of instantly pulling up/down a carrier to a baseband without using an IF. To be exact, although a conventional communication method has to employ this 0-IF method, it chooses a method without using the IF because of several problems including channel selectivity. That is, the 0-IF method has many problems that have to be overcome.
However, the method using 0-IF has significant advantages of low production cost, light weight and system unification since a surface acoustic wave filter (SAW filter) is not needed.
Thus, efforts to use 0-IF in mobile communication, for example, a global system for mobile communication (GSM), are being made, and therefore a number of mobile communication systems employing the 0-IF method has recently increased.
Particularly, a direct conversion receiver (DCR) adopting the 0-IF method or the direct conversion method is easily formed in one integrated circuit due to a simple circuit configuration, as compared to the super heterodyne receiver, and thus may be manufactured in a small size and at a low cost.
However, the DCR has some disadvantages. One problem that the DCR has is a second intermodulation distortion (IMD2) provided in the mixer. The IMD2 is caused by the fact that a mixer generally uses a nonlinear active device.
That is, the mixer may generate nonlinear distortion, which may be harmonic distortion generated in integer times of an input frequency, or the IMD2 in which different components are multiplied in order to form a new component.
In general, an LO signal is efficiently operated as a square wave by a switching operation of the mixer. Such a switching operation provides many advantages including low noise, high gain, sensing device's mismatching and deformation, sensing an exact LO length, and simple design.
However, such a switching operation has a disadvantage of producing odd-order mixing products (OMPs). Here, the OMP is generally defined by a product obtained by multiplying two different input odd-numbered harmonics.
This may be overcome by employing a sub-harmonic mixer to the DCR, wherein the mixer may overcome this problem by isolating the RF signal and the LO signal due to a limited separation state in-between paths thereof.
For example, in the conventional sub-harmonic mixer, the RF signal and the LO signal are input to a combiner, and the LO harmonic and the RF generated in the active device are mixed to thereby be directly input to an analog to digital converter (ADC). However, in this case, since the double LO (2LO) signal and the RF signal have frequency components, which are closely adjacent to each other, particularly in a millimeter wave band, isolation between the 2LO and the RF is difficult to ensure.
As an example of the conventional sub-harmonic mixer using a Gilbert cell mixer, the isolation characteristic between the 2LO and the RF may be improved, which results in poor manufacturing yield of a semiconductor integrated circuit due to a large number of transistors, high power consumption and complicated circuit configuration.
Also, in Korean Patent Publication No. 10-2005-0055733, as an example of the conventional sub-harmonic mixer, an IF signal formed in a mixer 1 is mixed with an LO signal phase-delayed by +90 and −90 degrees in a mixer 2 using a 45-degree phase shift circuit in a rear end of the LO signal and a front end of the mixer, thereby forming a baseband signal. This also has disadvantages of high power consumption and poor yield due to a number of the transistors.